This invention relates to a protection circuit for protecting a chargeable electric cell such as a lithium ion cell by detecting overcharge or overdischarge of the cell.
Among various types of chargeable electric cells, a lithium ion cell is particularly weak against overcharge or overdischarge. In this connection, it is essential to provide a protection circuit for detecting an overcharged condition and an overdischarged condition to inhibit a charging operation and a discharging operation, respectively. For this purpose, the protection circuit comprises two kinds of voltage detection circuits including an overcharge detection circuit and an overdischarge detection circuit.
A cell unit is formed by a combination of the lithium ion cell or cells and the protection circuit. In the cell unit, for example, two lithium ion cells (hereinafter simply called cells) are connected in a cascading manner with respect to each other. Each of the cells is connected in parallel to an overcharge detection circuit and an overdischarge detection circuit. The number of cells may be one or three or more. In any event, each cell is connected in parallel to one overcharge detection circuit and one overdischarge detection circuit. A charging device for carrying out a charging operation is connected between an input terminal and an output terminal of the cell unit. In addition, a load for use in a discharging operation is connected between the input terminal and the output terminal of the cell unit.
Each overcharge detection circuit is provided for comparing a terminal voltage of each cell with a first reference voltage. When the terminal voltage is higher than the first reference voltage, it is judged that the cell is overcharged. At that time, a first switch for switching electrical connection and disconnection of the charging device is turned off. Thus, the charging device is electrically disconnected from the cells to inhibit the charging operation.
On the other hand, each overdischarge detection circuit is provided for comparing the terminal voltage of each cell with a second reference voltage. When the terminal voltage is lower than the second reference voltage, it is judged that the cell is overdischarged. At that time, a second switch for switching electrical connection and disconnection of the load is turned off. Thus, the load is electrically disconnected from the cells to inhibit the discharging operation.
In each of the overcharge detection circuits and the overdischarge detection circuits, comparison of the voltages is carried out by the use of a comparing circuit. The comparing circuit comprises a reference voltage generating Zener diode connected to a constant-current source, two bleeder resistors, and a comparator.
The accuracy in detection or judgement of each of the overcharged and the overdischarged conditions by the overcharge and the overdischarge detection circuits is dependent upon a comparison accuracy of the comparing circuit. In turn, the comparison accuracy is determined by an accuracy of the reference voltage, a resistance ratio of the two bleeder resistors, and an offset voltage of the comparator.
In the case where the comparing circuit is implemented by an IC, a band gap Zener diode is often used as the reference voltage generating Zener diode.
The comparing circuit implemented by the IC is required to have a high comparison accuracy. Practically, due to various factors in the manufacturing process, the comparison accuracy practically achieved in every individual IC fluctuates beyond an allowable range. In this connection, various values in every IC as manufactured are adjusted by a trimming technique as well known in the art so as to make the comparison accuracy of every IC coincide with a designated value. In addition, it is proposed to use a circuit for reducing fluctuation arising in the manufacturing process. Furthermore, various improvements have been made as regards patterning.
For example, in order to achieve a high accuracy in matching of differential pair transistors composing the comparator, transistor patterns are increased in size. In addition, the transistor patterns are arranged in an improved arrangement, for example, in an alternate arrangement so as to reduce the influences of configuration and stress. On the other hand, resistor patterns are formed to have a uniform length and are a uniform width and arranged in an improved arrangement.
It is noted here that the resistor patterns are preferably widened in order to increase the accuracy of the resistance ratio, which is one of those the factors determining the comparison accuracy. This means that the resistor patterns have a high resistance. In addition, in order to reduce a current consumption, a high resistance is required also. If the resistor patterns are increased in width so as to obtain such a high resistance, a greater area is required within the IC. However, in the cell unit of the type described, it is general that reduction in current consumption is more important than reduction in size. In this connection, the comparing circuit of the conventional protection circuit uses a high resistance in order to reduce a current consumption as a first priority.